Light-emitting diodes in which an LED chip emits primary light with a first wavelength (for example blue light) on a light-emitting surface and the light-emitting surface is covered with silicone, with which a first luminophore and a second luminophore are admixed as fillers, are known. The first luminophore converts the primary light partially into a first secondary light with a longer wavelength (for example into green light) and the second luminophore converts the primary light partially into a second secondary light with a different even longer wavelength (for example into red light). The light-emitting diode consequently emits a mixed light including a proportion of the primary light, a proportion of the first secondary light and a proportion of the second secondary light, for example a white or whitish (for example warm-white) mixed light.
In this case, it is disadvantageous that the first secondary light with the shorter wavelength may be converted partially by the second luminophore into the second secondary light with the wavelength longer than it. This multiple wavelength conversion process leads to a quantum efficiency loss as well as to a less attractive spectrum with a low color rendering index of typically about 80. Furthermore, the refractive index difference between the luminophores and the silicone leads to further absorption losses. The low thermal conductivity of silicon, about 0.1 to 0.2 W/(m·K) furthermore leads to a high temperature of the luminophores, which limits a possible luminous flux density, reduces the conversion efficiency (particularly of nitridic or nitride-ceramic red luminophore) and can lead to degradation of the luminophores.
The problem of multiple wavelength conversion can so far be solved in lighting devices having a plurality of light-emitting diodes by a first subgroup of the light-emitting diodes including a luminophore layer having only the first luminophore, and a second subgroup of the light-emitting diodes including a luminophore layer having only the second luminophore. This allows a high color rendering index of about 90. A disadvantage in this case is that a plurality of light-emitting diodes are required therefor and a color homogeneity is lower, in particular at large angles with respect to the main emission direction.
The problem of the poor thermal conductivity of the luminophore layer can so far be solved by using ceramic luminophore layers. Such ceramic luminophore layers include a ceramic base material to which at least one activator (often a rare earth such as Ce or Eu) is added. By addition of an activator, the capacity for wavelength conversion is imparted to the ceramic luminophore layer. The ceramic base material (without activator) is typically transparent or translucent.
The ceramic luminophore layers may in particular be produced in a similar way to other ceramic bodies, for example by sintering preshaped green bodies, and thus consist essentially of the ceramic luminophore (possibly with small amounts of sintering auxiliaries or the like remaining). The use of ceramic luminophore layers has the advantage that they allow efficient wavelength conversion (for example at least 10% more efficient for wavelength conversion to green or yellow), are thermally conductive to a high degree (at about 10 W/(m·K)), are mechanically stable and exhibit little light attenuation. A disadvantage is that although green and yellow ceramic luminophores can be produced relatively simply and economically, red ceramic luminophores cannot.